xen/arm: ffa: fix build with clang/ATfE

Clang 22.x.x or Arm Toolchain for Embedded 22.x.x reports:

In file included from arch/arm/tee/ffa.c:75:
arch/arm/tee/ffa_private.h:439:17: error: 'used' attribute ignored on a non-definition declaration [-Werror,-Wignored-attributes]
  439 | extern uint32_t __ro_after_init ffa_fw_version;
      |                 ^

There is a fix for clang 16 in the git history:
a1a1e2b7dc ("xen/arm: ffa: fix build with clang")
which was broken by the following commit:
a90376c6ad ("xen/arm: ffa: Add FF-A 1.2 endpoint memory access descriptors")

Remove __ro_after_init from the extern declaration of ffa_fw_version.
The attribute is already present on the definition in ffa.c, which is
sufficient and correct.

Fixes: a90376c6ad ("xen/arm: ffa: Add FF-A 1.2 endpoint memory access descriptors")
Signed-off-by: Dmytro Prokopchuk <dmytro_prokopchuk1@epam.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>

xen/arch/arm/tee/ffa_private.h

@@ -436,7 +436,7 @@ struct ffa_ctx {
 };
 
 extern DECLARE_BITMAP(ffa_fw_abi_supported, FFA_ABI_BITMAP_SIZE);
-extern uint32_t __ro_after_init ffa_fw_version;
+extern uint32_t ffa_fw_version;
 
 extern struct list_head ffa_ctx_head;
 extern rwlock_t ffa_ctx_list_rwlock;